The present invention relates to a liquid distributor and in particular, but not necessarily limited to, a liquid distributor for distributing a liquid substantially uniformly across a liquid-vapour separation column located on an off-shore floating platform such as a ship.
Liquid distributors are used to distribute liquid uniformly over an area. Uniform distribution is important for efficient separation, especially if the separation is thermodynamically difficult.
By way of an example, liquid distributors are used to uniformly distribute a cryogenic liquid over the packing in the cryogenic distillation column of an air separation unit (or “ASU”). Uniform distribution is ideal. However, this is very difficult to achieve in practice and, thus, most commercial column packings are able to redistribute the liquid to some extent if the distribution is not uniform. Therefore, in most commercial applications, some non-uniformity of liquid distribution is acceptable.
In the prior art, there are two main types of liquid distributor, an open (e.g. channel) distributor and a closed (e.g. pipe) distributor. In the open distributor, the liquid is usually distributed under gravity via apertures (normally holes) located in the bottom of a series of open channels. The liquid is in contact with the surrounding vapour and, thus, the distributor cannot be pressurised. The closed distributor is usually made from pipes. The liquid is not in contact with the surrounding vapour and can be pressurised.
A typical design of open channel distributor has two sections, a primary distributor in fluid communication with a secondary distributor. The primary distributor reduces the velocity of the incoming liquid and evens out the flow. It may be in two-way fluid communication with the secondary distributor. The secondary distributor is responsible for the final distribution of the liquid, for example, on to the packing of the column of an ASU. All parts of the secondary distributor are in two-way fluid communication with each other. The liquid usually leaves the distributor via one or more apertures and relies solely on liquid head, i.e. gravity, to make the liquid flow. To achieve uniform distribution, the liquid head must be substantially constant when flow division occurs, especially in the secondary distributor above the final distribution points.
In practice, there is always a slight hydraulic gradient along the distributor in order for the liquid to flow from one point to another. Distribution points at the outer edges of the secondary distributor, furthest from the liquid entry point, have slightly less head then those points nearer the middle. This difference can be significant on very large columns. In addition, if the distributor is not in horizontal alignment, non-uniform distribution of the liquid will be observed.
In typical ASUs, the accuracy of liquid distribution required is very high, often less than a 5% standard deviation across the distributor, in order to obtain high purity products from thermodynamically difficult separations. This results in a need to build expensive high quality distributors. However, some off-shore as well as land-based applications could require only low purity products and, thus, the separation becomes thermodynamically easier and the quality of liquid distribution is not as critical.
When an open channel distributor is not mounted horizontally or is mounted in a moving plane, for example onboard a ship, then the liquid distribution will vary as the distributor moves out of horizontal alignment. In this situation, the liquid is not evenly distributed within the distributor. On a large column, this can cause a significant change in the uniformity of distribution, in particular at turndown when the level of liquid in the distributor is reduced and the resultant flow rates become more sensitive to motion or to a permanent tilt.
One way to mitigate this problem is to use a closed distributor and to use a pump to move the liquid through the distributor under pressure rather than use gravity. The higher pressure head from the pump reduces the impact of the change in liquid head due to gravity as the distributor tilts. However, the use of a pump increases the capital and operating costs of the separation process, often requires a back-up pump and uses electricity in a possibly hazardous environment. Alternatively, the liquid head due to gravity can be increased by using a higher liquid level, thereby reducing the change in liquid head observed as the distributor tilts. However, this increases the distance between the distributor and the centre of rotation which magnifies the adverse effects of the motion of the sea on the liquid distributor. Therefore, a balance between these two factors needs to be reached. Further, available space is usually limited on board a ship or off-shore platform and, thus, the size of the apparatus is restricted.
U.S. Pat. No. 4,565,216 (Meier; published on Jan. 21, 1986), U.S. Pat. No. 6,294,053 (Darredeau; published on Sep. 25, 2001) and U.S. Pat. No. 6,395,138 (Darredeau et al; published on May 28, 2002) each discloses liquid distributors for liquid-vapour separation columns located on floating platforms. Each liquid distributor has a primary distribution zone in fluid communication with at least one secondary distributor and the or each secondary distributor distributes liquid over the upper surface of packing within the column through a plurality of apertures. However, none of these references refers to a specific relationship between the two apertures that are furthest apart in the or each secondary distributor, a specific relationship between the arrangement of the or each secondary distributor and the column packing or the importance to distributor design of the type of separation occurring in the column.
A cryogenic distillation column is usually packed with structured packing. Typically, the packing consists of a plurality of corrugated, perforated sheets of metal which are arranged vertically in parallel throughout the column. However, it is known to pack a column in sections with each section consisting of a number of these packing sheets arranged vertically and in parallel but which, as a section, are orientated at an angle to a neighbouring column packing section. For example, U.S. Pat. No. 5,984,282 (Armstrong et al; published on Nov. 16, 1999), the disclosure of which is incorporated herein by reference, discloses a number of arrangements of structured packing for liquid-vapour contact columns. However, only one liquid distributor is disclosed for use with every arrangement of structured packing. In other words, the distributor is not designed with a specific arrangement of packing in mind. The distributor in question is that disclosed in U.S. Pat. No. 6,149,136.
U.S. Pat. No. 6,149,136 (Armstrong et al; published on Nov. 21, 2001) discloses a liquid distributor comprising a header tank in fluid communication with rows of reservoir cells in a liquid distribution plate located above the structured packing of a ship-board cryogenic distillation column. The distribution plate is a circular disc covering the whole cross section of the column and has identical bores of square cross section formed therein in uniform criss-cross rows. Alternate bores (vapour riser passages) are open at both the top and bottom to permit free flow of vapour therethrough and the remaining bores (reservoir cells) are blind being closed at their bases except for a central hole permitting limited flow of liquid therethrough.
U.S. Pat. No. 6,338,774 (Lehman; published on Jan. 15, 2002) discloses an open liquid distributor for use with an air distillation column on a floating platform or a barge. The distributor works by distributing liquid from a point in a receptacle to a point on the packing of the column that is not in vertical alignment with the point in the receptacle. One of the embodiments of the distributor comprises a two-stage distribution configuration. The first stage is an upwardly opening receptacle having a number of openings in the bottom of the receptacle that are disposed in a single crown adjacent to the peripheral wall. At a lower level is located a second stage for local distribution of the liquid comprising a number of regularly spaced, upwardly opening secondary receptacles, each having a perforated bottom. Each secondary receptacle has a smaller diameter then the first stage receptacle and is supplied by two tubes, themselves supplied respectively by two diametrically opposed openings in the first stage receptacle. In this way, each secondary receptacle should continue to be supplied by the first stage receptacle irrespective of the inclination of the distributor.
There is a need for an improved distributor that is less costly to manufacture and is still effective in maintaining an acceptably uniform distribution of liquid while the distributor is moved out of horizontal alignment. The primary objective of embodiments of the present invention is to distribute liquid in a sufficiently uniform manner, possibly under pressure, on to the top layer of packing such that an effective separation can be achieved despite movement or permanent tilt of the distributor. In addition, there is a need for an improved distributor for use with columns having awkward or complicated arrangements of multiple sections of structured packing. Another objective of embodiments of the present invention is to distribute liquid in a sufficiently uniform manner, possibly under pressure, on to the top layer of such packing such that separation can be achieved regardless of the packing arrangement. In addition, it is desirable that the distributor has low capital and operating costs and is easy to build.